Repeater terminal



Aug. 17, 1965 z. G. LYON REPEATER TERMINAL 2 Sheets-Sheet l Filed March23, 1962 AGENT 2 Sheets-Sheet 2 Filed March 23, 1962 United StatesPatent O 3,2t3l,591 REPEATER TERMNAL Zeno G. Lyon, Plainfield, Nd.,assigner to Intemational Telephone and Telegraph Corporation, Nutley,NJ., a corporation of Maryland Filed Mar. 23, 1962, Ser. No. 181,994 17Claims. (Cl. S25-i3d This invention relates to multiplex communicationsystems and more particularly to a repeater terminal for multiplexcommunication systems of the frequency division type.

Frequency division multiplex communication systems operable over a longdistance, and employing one or more repeater terminals, have in the pastresulted in a pro-hibitive accumulation of distortion of the transmittedsignal due to the repeated demodulation to baseband and remodulationfrom baseband at the repeater terminals. lt

was primarily for this reason `that intermediate frequency coupledrepeater terminals have been resorted to in long haul, frequency`division multiplex communication systems. in this type of repeaterterminal, the intermediate frequency signal of the terminal receiver iscoupled directly to the intermediate frequency portion `of the terminaltransmitter without demodulation to baseband and remodulation frombaseband. However, when the need arises for extraction or insertion ofchannel signals at a repeater terminal, it has been a common practice todemodulate and remodulate the entire video signal with the attendantdistortion. In the demodulation and remodulation process, the principalproblem arises from the nonlinearity of the demodulator and modulator.In present practice, the distortion of the e components is in the orderof 60 db (decibels) as measured by noise loading tests.

in a cop-ending application to W. L. Glornb entitled, Multiplex RepeaterTerminal, Serial No. 4l,4t}3, led July 7, B160, now US. Patent No.3,180,938, and assigned to the same assignee as the present application,a system has been disclosed which provides drop-and-insert facilitiesand yet eliminates distortion of the signal due to the dernoduiating andremodulating process of the prior art arrangements. The system disclosedin the aboveidentified copending application employs a phase detector orcomparator coupled to the output of the terminal receiver, and afrequency modulated oscillator coupled to the terminal transmitter andalso to the second input of the phase comparator. The output of thephase comparator provides a signal proportional to the baseband of thereceived signal which is coupled to the frequency modulated oscillatorfor frequency control thereof in a manner to cause the output signal ofthe frequency modulated oscillator to follow the input signal to thephase comparator from the terminal receiver. ln the feedback loop fromthe output of the phase coniparator to the frequency control element ofthe frequency modulated oscillator is inserted a band rejection lter topass all those channels in the baseband or" the received signal otherthan the frequency segments or channels which are desired to e droppedat the repeater terminal. The dropped channels can be extracted from theoutput of the phase comparator prior to its application to the bandrejection filter and the signals to be inserted in the vacated frequencysegrnents can be inserted prior to the application of the signal to thecontrol element of the frequency modulated oscillator. It has been foundthat, while under certain circumstances the system of the copendingapplication operates satisfactorily, there are situations whereinstability can develop in the feedback circuit. This is due to the factthat the equivalent open loop phase shift of the feedback circuit fromthe output of the phase comparator to the input of the frequencyPatented Aug. l?, lll' modulated oscillator is always a minimum of 9()degrees lagging. The added lter, the band rejection filter, will causeinstability if this filter ever produces as much as 9G degrees laggingphase shift while the `open loop gain is unity or greater. A possibilityof this instability is apparent when it is considered that the filter inthe feedback loop must have a large phase slope if it is to have a largeamplitude slope. Thus, there are certain conditions w ich could causeinstability in the repeater terminal arrangement of the copendingapplication.

Therefore, it is an object of this invention to provide an improvedrepeater terminal retaining the advantage of the above-identifiedcopending application and eliminating the possibility of instability inthe circuit arrangement.

A feature of this invention is the provision of a repeater terminal fora communication system, such as a frequency division multiplex system,transmitting a composite signal including a plurality of distinctfrequency segments, such as frequency spaced signal channels cornprisinga receiver for the composite signal, a transmitter for the compositesignal, a network including two signal paths coupled in shuntrelationship, each of the paths including a source of signal ofpredetermined frequency cooperating to product a resultant signal havinga frequency predetermined-ly related to the frequency of the compositesignal, means coupled to the receiver and the network to control thefrequency of both the signal sources to cause the frequency of theresultant signal to follow the frequency of the composite signal, andmeans to couple the transmitter to the network.

Another feature of this invention is the provision of components in thetwo signal paths of the network to provide the two signal paths withcomplementary amplitudeversus-phase characteristics.

Still another feature of this invenion is the provision of a phasecomparator responsive to the received composite signal and the resultantsignal at the output of thev network to control the frequency of afrequency modulated oscillator disposed in each of the paths with abandpass filter bein" coupled to the input of one of the frequencymodulated oscillators having given amplitudeversus-frequency andphase-vers-us-frequency characteristics, hereinafter referred to asamplitude and phase characteristics, and a band rejection filter coupledto the input of the oscillator of the other path having amplitude andphase characteristics complementary to the given amplitude and phasecharacteristics.

Still a further feature of this invention is the extraction of thesignals of the frequency segment for drop channel purposes at the inputof the oscillator included in the path including the bandpass filter orat the output of this oscillator.

Still another feature of this invention is the provision of meanscoupled to the path including the band rejection lter at the output ofthe frequency modulated oscillator in this path to insert the signal tobe disposed in the frequency segment removed at the terminal.

Still a further feature of this invention is the provision of a mixerresponsive to the resultant signal output of the network and thereceived composite signal and a frequency discriminator coupled to themixer to produce a signal to control the frequency of the signal sourceof the two paths of the network to cause the frequency of the resultantsignal to follow the frequency of the composite signal.

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

FiG. 1 is a schematic diagram in block form of a communication systememploying a multiplex repeater terminal following the principles of thisinvention;

o FIG. 2 is a schematic diagram in block form illustrating anotherarrangement for the equipment between lines A-A and B-B of the system ofFIG. 1;

FIG. 3 is a schematic diagram in block form of still --anotherarrangement for the equipment between lines A-fA and B-B of the systemof FIG. l; and

FIG. 4 is'a schematic diagram in block form of another embodiment forthe equipment between lines C-C and B--B of FIG. l. Y Referring to FIG.1, there is illustrated therein in block diagram form a communicationsystem following the principles of thisI invention. For purposes ofexplanation, a frequency division multiplex system incorporating therepeater terminal of thisinvention will be described.V It is to beobserved, however, Vthat any composite signal having a multiplicity ofdistinct frequency segments may be operated on by the repeater terminalof this invention. The baseband signal of ,a frequency divisionmultiplex system is a multiplex signal having the usual character forfrequency division multiplex, namely, a plurality of sub-carrier signalsseparated one from the other to provide signal channels with each of thesubcarrier :signals being frequency modulated by intelligence to betransmitted. The baseband signal is then operated on to translate thefrequency spectrum thereof to the appropriate frequency region forpropagation in the communication medium. The frequency translatedbaseband may be propagated by frequency modulating a radio frequencycarrier. For instance, the baseband signal may be provided at terminal 1for transmission to terminal 2 along a communication medium includingtherein repeater terminal 3 which in certain instances may be a branchterminal repeater station receiving intelligence signals from apropa-gation path which makes an .angle with the propagation pathbetween terminals 1 and 2. Repeater terminal 3 .also may generatesignals to enable communication between terminal 3 and terminal 2.

It should be noted that the communication system outlined hereinabove isby way of example only, since ter- -m'inals 1 and 2 could be repeaterterminals and include the same components as terminal 3 and there couldbe more repeater terminals between either terminal 1 and terminal 3 orterminal 3 and terminal 2. Also for the sake `of simplicity, thecommuni-cation system is illustrated `as being a one-way communicationsyst-em. It is obvious that two-way communication may be obtainedbetween terminals 1 and 2 through terminal 3 by providing a secondcommunication path between terminals 2 and 1 via terminal 3 with theappropriate duplication of equipment necessary to carry on this two-waycommunication. If, of course, proper consideration is taken, the onlyduplication would be necessary in the equipment between lines A-A andB-B of terminal 3.

VTurning now to the description of repeater terminal 3, a receiver 4receives the multiplex signal from terminal 1 and a transmitter 5transmits the multiplex signal from terminal 3 to terminal 2. Receiver 4is illustrated as including an antenna 6, a radio frequency (RF)amplifier 7 -coupled to a heterodyning circuit including mixer 8 andoscillator 9 to produce an intermediate frequency (IF) versi-on of thereceived RF multiplex signal for application to IF amplifier 10.Transmitter is illustrated as including a heterodyning circuit includingmixer 11 and oscillator 12 to raise the IF signal output of network 13to the RF region for application to RF amplifier 14 and, hence, toantenna 15 for propagation to terminal 2.

Network 13 includes two signal paths 16 and 17 coupled in shuntrelationship, each of the paths 16 and 17 including a source of signalof predetermined frequency cooperating to produce a resultant frequencyhaving a frequency predeterminedly related to the frequency of themultiplex signal at the output of amplifier 10. Means illustrated in theembodiment of FIG. 1 as phase comparator or detector 1S, responds to theoutput of ampli- 4. fier 1t) and the output of network 13 to produce asignal to control the frequency of both the signal sources of paths 16and 17 to cause the frequency of the resultant signalV to follow thefrequency of the multiplex signal at the output of amplifier It).

More specifically, path 16 includesbandpass filter 19 having a passban-d characteristic to pass the predetermined frequency segment orsegments, such as signal channel or channels, of the frequency multiplexsignal to a frequency modulated oscillator 20 whose output is coupled toone input terminal of mixer 21. Path 17 includes band rejection filter22 having a pass band characteristic complementary to the pass bandcharacteristic of lter 19 to reject the frequency segments passed byfilter 19 and pass the other frequency segments. The signal output of-filter 22 is then coupled to the frequency modulated oscillatorV 23whose signal outputis coupled to the other input terminal of mixer 21.The signal output of mixer 21 is the resultant signal at the output ofnetwork 13 `and is coupled to phase comparator v1li to produce a controlsignal proportional to the phase difference between the signal output ofampli-fiel 1t) and the resultant signal output of mixer Z1. Thedifference or control signal at the output of phase comparator 18 iscoupled to the input of network 13, passed through the bandpass filters19 `and 22 and, hence, to oscillators 20 and 23, resulting in amodulation of oscillator 20 by those frequency segments passed by filter19 and a modulation of oscillator 23 by those frequency segments passedby filter 22. The output signals of oscillators 2t? andV 23 are coupledto mixer 21 whose output is the resultant signal coupled to phasecomparator 13 for comparison with the signal output of amplifier 10.

The control signal 'at the output of phase comparator 1'8 will be thebaseband of the IF signal at the output of amplifier 10 if thefrequencies of oscillators 20 and 23 are selected to provide a meanfrequency for the resultant signal at the -output of mixer 21 equal tothe mean frequency of the IF signal at the output of lamplilier 1t).This will be apparent from the following discussion. Consider first thatthe IF signal at the output of amplifier 10 is not modulated. Thus, withthe frequency of oscillators 211 and 23 selected as stated above, phasecomparator 18 will not produce a control signal since the frequency ofthe IF signal and the resultant `signal are identical. Consider now thefirst instant that frequency modulation is pres-ent on the IF signal.Phase comparator 18 will produce a control signal which is proportionalto the phase difference between the IF signal and the resultant signal.It will be observed that the phase difference between the signalscompared is due to the frequency modulation and, therefore, the controlsignal is proportional to the modulating signal or baseband. Thiscontrol signal is coupled to network 13 to frequency modulateoscillators 2t) 'and 23 as above described. Under this condition, theresultant signal will now have the same frequency modulation thereon aswas present onV the IF signal. During the next instant of time, theresultant signal modulated in accordance with the modulation of the IFsignal during the first instant will be compared with the IF signalmodulated differently than during Ythe first instant. The control signalwill 'be proportional to the phase difference between the two signals atthis instant of time and will, thus, cause the resultant signal toassume the modulation present on the IF signal. This comparison processis continued on an instantaneous basis. Thus, from the foregoing it isapparent that the frequency of the resultant signal lags the frequencyof the -IF signal or, in other words, follows the frequency of the IFsignal. It is also apparent that the control signal output is thebaseband of the IF signal.

To prevent the development of instability in the feedback loop or phaselock loop including paths V16 and 17 in accordance with this invention,the amplitude and phase characteristics Vof fil-ters 19 and 22 aredesigned to be,

complementary yand the resultant signal coupled from the output of mixerEl is the whole baseband of the signal coupled from amplifier it?.

To provide drop channel facilities, it would be possible to couple theutilization device 2ddirectly to the output of bandpass filter i9, sincethis filter passes only those channels or frequency segments which aredesired to be utilized at repeater terminal 3. Utilization device 24 canbe .a speaker or other device at 4the repeater terminal 3 itself, or maybe the appropriate modulation equipment for propagation of the droppedchannels from terminal 3 to a branch propagation medium yand branchterminal (not illustrated). An alternative arrangement for dropping theselected channel or channels may be obtained by moving the ganged switch25 to its other position so as to couple 4a discriminator 26 to theoutput of oscillator 21d, thereby providing `a frequency modulatedversion of the frequency channel or channels passed by filter l?.Discriminator Z6 `frequency dernodulates the frequency modulatedchannels passed by filter E9 with the output of discriminator 26 beingcouple-d to bandpass filter 27 to clean up the edges of the droppedsignal channels and, hence, to utilization device Zd which may be thesame type of utilization device described hereinabove with respect toutilization device Zf-i. lt should 'be understood that utilizationdevice 24 may include -a bandpass filter coupled to the input thereof toIclean up the signals coupled thereto, such as is accomplished bybandpass filter 27.

Since the signals of the channels selected by bandpess filter 19 lare tolbe utilized in terminal 3, or in a further branch terminal coupled vtoterminal 3, the selected dropped channels should be removed from thebaseband so that they are not coupled to transmitter 5 and, hence, toterminal 2. To accomplish this end, band rejection filter 2?. isdesigned to have a rejection band substantially identical with the passband of fil-ter i9 to remove the channel or group of channels dropped atterminal 3. As mentioned hereinabove, the amplitude and phasecharacteristics of rejection filter 22 should be complementary to filteri9 to remove the possibility of instability being deeloped in thefeedback loop of network i3. By removing the dropped channel orchannels, the baseband coupled to oscillator 23 for modulation thereofincludes one or more signal channels (frequency bands) which have beeneradicated. lt is now possible to insert in the vacant signal channel orchannels, signals originating at terminal 3 or received at terminal 3from a branch terminal, for coupling to terminal 2. This maybeaccomplished in one arrangement by coupling mixer 33 to path t7 at theoutput of oscillator 23. Oscillator provides a subcarrier signal havinga frequency disposed in the baseband frequency range so the modulatingsignal of source 3d can be modulated in modulator 36 on the subcarriersignal of oscillator 3d to occupy the eradicated signal channel lorchannels contained in the base band at the output o-f oscillator 25. Theoutput of mixer would then be coupled to the input of mixer to permittransmission of the original baseband signals plus `the signals inser edat terminal 3 to terminal 2.

Since, as pointed out hereinabove, the resultant signal of mixer 21follows the IF signal at the output of amplifier 19, the output signalof oscillator Z3 will likewise 'follow the frequency of the il: signalat the output of amplifier it) for those portions of the signal at theoutput of amplifier 14) which are passed by rejection filter 22. Thus,there actually is no demodulation of the signal at the output ofamplifier i@ prior to being coupled to the transmit-ter forrepropagation, thereby eliminating the distortion heretofore present dueto the non-linearity of the demodulator and modulator. Thus, thereresults a drop-and-insert repeater terminal which eliminates thepreviously experienced accumulation of distortion in repeater terminaloperation. AFurther, since bandpass filter l? and band rejection filter22 have complementary amplitude and phase characteristics and the signalcoupled from the outai Jar 6 put of mixer 2l includes all the ban-dbasecomponents for comparison against the baseband components applied fromthe output of amplifier lfb, the possibility of instability in the phaselock loop of this invention is substantially eliminated.

To summarize, the entire baseband operates to modulate two oscillatorsto produce Ia resultant signal for cornparison with an IF signal derivedfrom the terminal receiver. The error signal at the output of4comparator its', which represents the frequency discrepancy and thedis- 'tortion o-f the frequency in network i3 relative to the receivedfrequency modulated signal, is fed back to the oscillators 2li Aand 23to correct the distortion and to provide .a signal capable of modulatingthe transmitter in accordance with the baseband signal. With thisarrangement the system functions essentially as a directly coupledintermediate frequency repeater since the transmitter IF carrier iscorrected in frequency deviation and distortion to agree with thereceiver IF carrier.

Referring now to FIG. 2, an alternative arrangement for the componentsbetween lines A-A and B-B of FIG. 1 is illustrated. As pointed outhereinabove, the output signal of oscillator 23, FIG. lJ includes thebaseband with the eradicated signal channel or channels. The outputsignal of oscillator 23 is coupled to phase comparator 3d. A network 37,lsubstantially identical with network f3, `FG. l, includes two signalpaths .38 and 39 coupled in shunt relation. Each of these paths includesa signal source which is controlled by the 4output of comparator 3osubstantially in the manner accomplished by network 13. Morespecifically, network 37 includes in path 38 a frequency modulatedoscillator di) having the control signal of comparator 36 coupled Itothe frequency control element thereof by means of bandpass filter if andpath 39 includes .a frequency modulate-d oscillator d?. Ihaving theVcontrol signal from comparator 36 coupled through a band rejectionl-ter 43 to the frequency control element of oscillator 42. By adjustingthe frequencies -of oscillators el? and 4Z to provide a resultantfrequency at the output of mixer Lt having -a mean frequency equal tothe mean frequency of the output signal of oscillator 23, la controlsignal can be developed in comparator 35 proportional to the phasedifference between the two signals to cause the difference signal tocontrol the frequency of oscillators di? and 42 in a manner to causelthe resultant signal at the output of mixer ltd to follow the signal atthe Ioutput of oscillator 23. Thus, a second phase lock loop is providedwhich will lock the resultant signal at the output of mixer dit to theoutput signal of oscillator 23. Now to insert the signal or signals tooccupy the eradicated channels yin the baseband lall that is necessaryis to provide an adder 45 in path 39 between the output of fil-ter 43and the frequency control element of oscillator 42. Adder 45 operates toadd `the signals from source 3S, properly disposed in the frequencyspectrum to occupy 4the eradicated frequency segments or channels, tothe output signal of lter 43. The frequency modulation on the outputsignal of oscillator l2 will include the baseband output of filter t3and the inserted sign-als. This signal would then be coupled to mixerlll of FlG. l for transmission to terminal 2. By selecting the frequencyof oscillators 4@ and d2 `as illustrated in FIG. 2, the output coupledto mixer lll of FdG. 1 will have the same frequency as the output ofamplifier lili. It would also be possible in this and the otherarrangements 'disclosed therein to select the frequencies of thefrequency modulated oscillators to shift the frequency of the signalcoupled to the transmitter to be different from the frequency of thesignal coupled from the receiver. With this `arrangement it would, ofcourse, be necessary to select the proper output (the sum or differencefrequency signal) from the mixer so that the resultant signal will have-a mean frequency equal to the mean frequency -of the received IFsignal.

Referring to FIG. 3, another arrangement for the equipment between linesA-A and B-B of FIG. l is illusis coupled to phase comparator 36 and theresultant signal at the output of network 37' is produced by frequencymodulating a single frequency modulated oscillator 47. The remainder ofnetwork 37 includes a first path 3S including a bandpass filter 4i forpassing the eradicated frequency band (signal channel) and a second path39 including band rejection filter 43- to pass all the signal componentsother than the components passed by` filter 41. The outputs of filters41 and 4'3' are coupled to an adderrand isolator 48 to add the signalsat the output thereof and to maintain these filters isolated one fromthe other. The output signal of adder and isolator 4S is coupled to thefrequency control element of oscillator i7 to control the frequencythereof so that the resultant signal coupled to phaseV comparator 35will follow the frequency of the signal coupled from oscillator 23, FIG.l. Thus, the difference signal or control signal is fed through paths 38and 39 and combined in adder and isolator 43 to control the frequency ofoscillator 47.. In this arrangement the channel signal or signals areadded to the baseband by employing an adder 49 which adds the signals ofsource 3S to the baseband provided the signals of source 3S have acenter frequency or center frequencies properly located in the frequencyspectrum of the baseband to occupy the eradicated frequency band.v Hereagain the circuit will lock up as in the previous arrangements tomaintain the output signal of oscillator 57 in step with the frequencyof the signal at the output of oscillator 23. The output signal ofoscillator 47 is coupled to mixer 11 for transmission to terminal 2.

In the embodiments of FIGS. l, 2, and 3 the Vcharacteristic compared inthe means to control the frequency of the two oscillators (phasecomparator 1S) has been the phase of the two signals with the differenceor control Signal maintaining the resultant signal at the output ofnetwork 13 in step with the input signal to phase comparator 1S. In theembodiment illustrated in FIG. 4, similar network components areemployed and the technique is substantially the same as that describedin connection with FIGS. l, 2, and 3 with the exception that thecharacteristic of the resultant signal and the received signal comparedis frequency variation. The embodiment illustrated in FIG. 4 can besubstituted for the equipment illustrated in FIG. l between lines C-Cand B--B. As in the embodiment of FIG. l, a network 51 Vis providedhaving twosignal paths coupled in shunt relation. The first signal pathincludes a frequency modulated oscillator 52 and a bandpass filter S3while the second signal path includes a frequency modulated oscillator54 and a band rejection filter 55. The output signals of oscillators 52and 54 are coupled to mixer 56 with the frequency of the two oscillatorsbeing arranged to provide a means frequency for the resultant signaloutput of mixer 56 which is in a predetermined frequency relationshipwith the mean frequency of the received signal coupled from amplifier tomixer 57. The output of mixer 5o is coupled to mixer 57 to produce abeat signal at the output thereof. The beat signal is coupled to limiter58 for amplitude control and, hence, to discriminator 59 which producesa control signal proportional vto the frequency difference between thesignal output of mixer 56 and the signal coupled from amplifier in. Thecontrol signal at the output of discriminator 59V is coupled to network5l and, hence, will control the frequency of oscillators 52 and S4 in amanner tomaintain the frequency of the resultant signal output of mixer56 in the predetermined frequency relationship with the frequency of thesignal applied from amplifier 1t). As in the embodiments previouslydescribed, bandpass filter 53 will have a-pass band characteristic topass a frequency segment (signal channel) or a plurality of frequencysegments, depending upon the signals to be dropped at the repeaterterminal 3. The band rejection filter S5 will have a complementary passband characteristic to that of filter 53 to pass all those otherfrequency segments that are not passed Aby filter 53 and to block thefrequency segments passed by filter 53. In addition, the amplitude andphase characteristics of these two filters are complementary. Thechannels to be dropped may be dropped by coupling discriminator 26 tothe output of oscillator 52 with the resultant detected frequencymodulated frequency segments being coupled to utilization device2liwhich can include a bandpass filter at the input thereof to cleanupthe detected baseband channel signals. To add the signal or signalsoriginating at terminal 3 or received at terminal 3 from branchterminals, adder di) is coupled to the output of oscillator 5J:- to addthe signal of source 35 having a carrier frequency properly disposed inthe frequency spectrum of the baseband to occupy the frequency segmentor segments of the channel signals dropped at terminal 3. The outputsignal of adder 6) is then coupled to mixer il and, hence, to terminal2. It should be further pointed out that the reinsertion arrangementsbetween lines A-A. and B-B of FIGS. l, 2, and 3 can be substituted forthe equipment between lines A--A and B-B of FIG. 4.

The repeater terminal arrangements described hereinabove have theadvantage of being physically simple and economical in that demodulatorsand modulators at baseband level are eliminated and, in addition, havethe advantage of being virtually distortionless. The fact that therepeater terminal of this invention is distortionless is because thenon-linearity of the modulationcharacteristic of the frequency modulatedoscillators is reduced by the loop gain of the phase or frequencycomparison circuit. Moreover, due to the complementary characteristicsof the bandpass and band rejection filters and, since the full basebandsignal is utilized for comparison purposes in the arrangements of thisinvention, a very high loop gain can be employed without instability.

While I have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationto the scope of my invention as set forth in the objects thereof and inthe accompanying claims. Y

I claim:

1. Arepeater terminal for a communication system transmitting acomposite signal comprising:

a receiver for said composite signal;

a transmitter for said composite signal;

a network including twosignal paths coupled in shunt relationship, eachof said paths including a source of signal of predetermined frequencycooperating to produce a resultant signal having a frequencypredeterminedly related tothe frequency of said composite signal;

means coupled to the output of said receiver and the loutput of saidnetwork to produce a control signal;

means to couple said control signal to the input of said network tocontrol the frequency of both said signal sources to cause the frequencyof said resultant signal to follow the frequency of said compositesignal; and

means to couple said transmitter to said network.

2. A repeater terminal for a communication system transmitting acomposite signal comprising:

a receiver for said composite signal;

a transmitter for said composite signal;

a network including two signal paths coupled in shunt relationshiphaving complementary amplitude and phase characteristics, each of saidpaths including a source of signal of predetermined frequencycooperating to produce a resultant signal having a frequencypredeterminedly related torthe frequency of said composite signal;

means coupled to the output of said receiver and the output of saidnetwork t-o produce a control signal; eans to couple said control signalto the input of said 9 network to control the frequency of both saidsignal sources to cause the frequency of said resultant signal to followthe frequency of said composite signal; and means to couple saidtransmitter to said network. 3. A repeater terminal for a communicationsystem transmitting a composite signal including a plurality offrequency segments comprising:

a receiver for said composite signal;

a transmitter for said composite signal;

a network including two signal paths coupled in shunt relationshiphaving complementary amplitude and phase characteristics, each of saidpaths including a source of signal of predetermined frequencycooperating to produce a resultant signal having a frequencypredeterminedly related to the frequency of said composite signal;

means coupled to the output of said receiver and the output of saidnetwork to produce a control signal;

means to couple said control signal to the input `of said network tocontrol the frequency of both said signal sources to cause the frequencyof said resultant signal to follow the frequency of said compositesignal;

means to couple said transmitter to said network; and means coupled toone of said paths to extract the signal of at least one of saidfrequency segments.

d. A repeater terminal for `a communication system transmitting acomposite signal including a plurality of frequency segments comprising:

a receiver for said composite signal;

a transmitter for said composite signal;

a network including two signal paths coupled in shunt relationship, eachIof said paths including a source Iof signal of predetermined frequencycooperating to produce a resultant signal having a frequencypredeterminedly related to the frequency of said composite signal;

a first means having given amplitude and phase characteristics disposedin one of said paths to pass at least a selected one of said frequencysegments;

a second means having amplitude and phase characteristics complementaryto said given amplitude and phase characteristics disposed in the otherof said paths to pass all said frequency segments other than saidselected frequency segments;

means coupled to the output of said receiver land the output of saidnetwork to produce a control signal;

means to couple said control signal to the input of said network tocontrol the frequency of both said signal sources to cause the frequencyof said resultant signal to follow the requency of said compositesignal;

means to couple said transmitter to said other of said paths; and

means coupled to said one of said paths to extract the signal of saidselected frequency' segments.

5. A repeater terminal for a communication system transmitting acomposite signal including ya plurality of frequency segmentscomprising:

a receiver ror said composite signal;

a transmitter for said composite signal;

a network including two signal paths coupled in shunt relationship, eachof said paths including a source of Signal of predetermined frequencycooperating to produce a resultant signal having a frequencypredeterminedly related to the frequency of said composite signal;

a first means having given amplitude and phase characteristics disposedin one of said paths to pass at least a selected one of said frequencysegments;

a second means having amplitude and phase characteristics complementaryto said given amplitude and phase characteristics disposed in the otherof said paths to pass all said frequency segments other than saidselected frequency segments;

means coupled to said receiver and said network to con: trol thefrequency of both said signal sources to cause .the frequency of saidresultant signal to follow the frequency of said composite signal;

means to couple said transmitter to said other of said paths;

means coupled to said one of said paths to extract the signal of saidselected frequency segments;

the output of said first means being coupled to the source of signal ofsaid one of said paths; and

said means to extract being coupled to said output of said first means.

A repeater terminal for a communication system transmitting a compositesignal including a plurality of frequency segments comprising:

a receiver lor sai-d composite signal;

a transmitter for said composite signal;

a network including two signal paths coupled in shunt relationship, eachof said paths including a source of signal of predetermined frequencycooperating to produce a resultant signal having a frequencypredeterminedly related to the frequency of said composite signal;

a first means having given amplitude and phase characteristics disposedin one of said paths to pass at least a selected one of said frequencysegments;

a second means having amplitude and phase characteristics disposed inthe other of said paths to pass all said frequency segments other thansaid selected frequency segments;

means coupled to said receiver and said network to control the frequencyof both said signal sources to cause the frequency .of said resultantsignal to follow the frequency of said composite signal;

means to couple said transmitter to said other of said paths;

means coupled to said one of said paths to extract the signal of saidselected frequency segments;

the output of said lirst means being coupled to the source of signal ofsaid one of said paths; and

said means to extract being coupled to the output of said source ofsignal of said one of said paths.

7. A repeater terminal lfor a communica-tion system transmit-ting acomposite .signal including .a plurality of frequency segments-comprisir a receiver for said composite signal;

a transmitter `for said composite signal;

a network including two signal paths coupled in shunt relationship, eachlof said paths including a source of signal of .predetermined frequencycooperating to produce a resultant signal lhaving a frequencypredeterminedly related yto the frequency of said oomposite signal;

:a first means having given .amplitude and phase characteristicsdisposed lin one of said paths to pars-s at least a selected one of saidfrequency segments;

a second means having amplitude and phase characteristics complementary'to said given amplitude `and phase characteristics disposed in theoth-er of said paths to pass all said frequency segments other than saidselected frequency segments;

means coupled to said receiver and said network to control the frequencyof `both .said signal sources to cause the frequency of said resultantsignal to follow the frequency of said composite signal;

means coupled to said other of said paths to insert a signal into saidselected Afrequency segments; and' means to couple the signal at theoutput yof said second means and the inserted signal to saidtransmitter.

A 'terminal :according to claim 7, wherein:

the output of said second means is coupled to the lsource of signal ofsaid other of said paths;

said means to insert is coupled to the output of said source of signallof said lother of :said paths; and

said tr-ansmitter -is coupled to the output of said means to insert. 9.A repeater terminal f-or a communication system transmitting lacomposite signal including `a plurality of frequency segmentscomprising:

=a receiver for said composite signal; a transmitter for sai-d compositesignal;

la network including two signal paths coupled in shunt relationship,

eac-h of said paths includinr a source of signal of predeterminedfrequency coopera-ting to produce a resultant signal having a frequencypredeterminedly related to the frequency yof said composite signal;

a first means having given amplitude and phase characteristics disposedlin Ione of said paths to pass 1at least Ia selected one of saidfrequency segments;

a second means having amplitude and phase characteristics complementaryIto said given amplitude and phase characteristics disposed in the otherof said paths to pass all said frequency segments other than saidselected frequency segments;

means coupled to said receiver and said network to control theyfrequency of :both said signal sources to cause the frequency of saidresultant signal to follow the .frequency of said composite signal; l

means coupled to said one of said paths Ito extract the signal of -saidselected frequency segments;

means coupled to said other of said paths to insert a signal into saidselected frequency segments; and

means to couple the signal at the output of said second lmeans and saidinserted signall to said transmitter.

10. A terminal according to claim 9, wherein:

the output of said ii-rst Imeans is coupled to the source of signal ofsaid one of said paths;

Y said means to extract is coupled to -said output. of said first means;

the output of said second means is coupled to the source of signal ofsaid other of said paths;

said means to insert is coupled to the output of said source of signalof said other of said paths; land said transmitter is coupled to theoutput of said means to insert.

11. A terminal according to claim 9, wherein:

the output of said iirst means is coupled to the source of sign-al ofsaid one yof said paths;

said means to extract is coupled :to the outputof said source of signalof .said one of said paths;

the output of said second lmeans is coupled to the source of signal ofsaid other .of said paths;

said -means to insert is coupled to the output of said source of signalof said other of said paths; :and

said 'transmitter is coupled to the output o-f said means to insert.

12. A repeater terminal for a communication system transmitting -acomposite signal comprising:

ia receiver `for said composite signal;

:a transmitter for said composite sign-al;

rst means to gener-ate Ia first signal having a first predetermined'frequenc second means to generate Ka second sign-a1 having 'a secondpredetermined frequency;

means common to said first and second generator means to combine saidfirs-t and second signals;

means coupled to said receiver and said common means responsive to saidreceived composite signal and said combined iirst and second signals toproduce Aa control signal proportional to lthe dierence Ibetween apredetermined characteristic thereof;

means to couple .said control signal to each of said iirst and secondgenerator means to control the frequency of said .ti-rst :and secondsignals to cause the frequency of said combined iirst and sec-ondsignals to follow the frequency of said received composite signal; and

means to couple said transmitter 'to at least one of said yfirst andsecond means. 13. A repeater-terminal .for :a communication systemtransmitting a composite signal including a plurality of 5 frequencysegments comprising:

.a receiver for said composite signal;

la transmitter for said composite signal;

a first series circuit including a band pass iilter to pass at least aselected one of said frequency segments land a first frequency modulatedoscillator to generi ate :a firs-t signal; Y

la second series circuit .including a band rejection filter to pass allsaid frequency segments other than said :selected frequency segments and-a second frequency modulated oscillator to generate 1a second signal;

common means coupled to the output of said first and second oscillatorsto combine said rst and second signals to provide a resultant :signalhaving a frequency equ-al to the frequency of said received compositesignal;

a phase comparator coupled to said receiver and said com-mon meansresponsive to said received composite signal and said resultant signalto produce a control sign-al proportional to the phase differencetherebetween; v

means to couple said control signal to each of said iilters'to controlthe lfrequency of -said first and second oscillators to cause thefrequency of lsaid resultant signal lto follow the frequency `of saidreceived compos-ite signal; i

means coupled to said iirst series circuit to extract the signal of saidselected frequency segments;

means coupled to sai-d second series circuit to insert a signal intosaid selected frequency segments; and

means to couple lthe signal .at the output of said band rejection iilterand the inserted signal to said transmitter.

A terminal according to claim 13, wherein said means to insert includes:

.a third series cir-cuit having a second handpass iilter to pass saidselected frequency segments and a third frequency modulated oscillatorto generate )a third signal; v

ya fourth series circuit having a second -band rejection tilter to passallV said lfrequency .segments other than said selected frequencysegments and a fourth frequency modulated oscillator to generate afourth signal;

a second common 4means coupled to the output of said third and lfourthoscillators to combine said third and fourth signal to :provide Iasecond resultant signal having :a frequency equal to the frequency ofthe signal output of said second oscillator;

fa second phase comparator coupled to the output of said .secondoscillator and said second common means responsive to said second`resultant signal and the output -signal of said second oscillator toproduce a second control signal proportion-al to the phase differencetherebetween;

`means to couple said second cont-rol signal to each of said secondlters to control the frequency of said third and fourth oscillators tocause the frequency of said second resultant signal to follow thefrequency of the output signal of said second oscillator;

means coupled to the output of said second band rejection filter toinsert said inserted signal; and

means coupling said transmitter to the output of said fourth oscillator.

1S. A terminal according to claim 13, wherein said means to insertincludes:

a -second band rejection tilter rto pass fall said frequency segmentsother than said selected vfrequency segments;

a second bandpass iilter to pass said selected frequency segments;

a third frequency modulated oscillator coupled in common to said secondiilters to :provide la second resultant sign-al having a `frequencyequal to the frequency of the output signal of said second oscillator;

insertion 'means coupled to the input of said second candpass ltil'tertto insert said inserted signal;

ya second phase comparator coupled to the output of said secondoscillator :and ithe output of said third yoscillator responsive to saidsecond resultant signal .and the output of said second oscillator toproduce a second control signal proportional Ito the phase differencetherebetween;

means to couple said second control signal 4to said insertion means Iandsaid .second band rejection lter to control the Afrequency of saidftliird oscillator to cause the frequency Aof 4said second resultantsignal Ito follow the frequency of the output signal of said secondoscillator; yand means coupling said `transmitter to lthe output of saidthird oscillator.

16. A repeater lterminal for a communication system transmitting acomposite signal comprising:

a receiver for .said composite signal;

a transmitter for said composite signal;

`a first frequency modulated oscillator to generate a nrst signal;

`a second frequency modulated oscillator -to generate a second signal;

means common to said rst and second generator means to combine saidfirst and second signals to provide a resultant lsignal having afrequency predeterminedly related to the frequency of said receivedcomposite signal;

a mixer means coupled Ito said receiver and said common means responsiveto said received composite signal and said resultant signal to produce abeat frequency signal;

a discrimina-tor coupled to said miner means Ito produce ya controlsignal proportional rto the `frequency difference between said receivedcomposite signal and said resultant signal;

means to couple said control signal to each of said filters to controlthe frequency of said irst and second oscillators to cause the frequencyof said resultant signal to follow the frequency of said receivedcomposite signal; and

one-s1 transmitting a composite signal including a plurality of 5frequency segments comprising:

la receiver for said composite signal;

a ltransmitter for s-aid composite signal;

a first series circuit including a bandpass iilter to pass at least aselected one of said frequency segments and a first frequency modulatedoscillator to generate `a yrst signal;

a second series circuit including a band rejection filter ito pass allsai-d frequency segments other than said selected frequency segments andia sec-ond frequency modulated oscillator to generate a second signal;

means common to the Aoutput of said first and second oscillators tocombine said first and second signals to provide a resultant signalhaving a frequency predeter-minedly related to the frequency Iof saidreceived composite signal;

.a mixer means coupled to said common means and said receiver responsiveto said received composite signal and said resultant signal to produce abeat frequency signal;

a discrimin-ator coupled to said mixer means to produ-ce a controlsignal proportional to the lfrequency difference ibetween `sai-dreceived composite signal and said resultant signal;

means to couple said con-trol sign-al to each of said lters to controlthe frequency of said rst and second oscillators to cause the yfrequencyof said resultant signal 'to follow the lfrequency of said receivedcomposite signal;

means coupled to said first series circuit to extract the signal of saidselected frequency segments;

means coupled to said second series circuit to insert a Isign-al intosaid .selected yfrequency segments; .andl

means Ato couple the signal at fthe output of said band rejection`filter .and the inserted signal to said trans- Imit-ter.

References Cited by the Examiner UNITED STATES PATENTS 2,907,874 10/59Halborson -179-170 DAVID G. REDINBAUGH, Primary Examiner.

1. A REPEATER TERMINAL FOR A COMMUNICATION SYSTEM TRANSMITTING ACOMPOSITE SIGNAL COMPRISING: A RECEIVER FOR SAID COMPOSITE SIGNAL; ATRANSMITTER FOR SAID COMPOSITE SIGNAL; A NETWORK INCLUDING TWO SIGNALPATHS COUPLED IN SHUNT RELATIONSHIP, EACH OF SAID PATHS INCLUDING ASOURCE OF SIGNAL O PREDETERMINED FREQUENCY COOPERATING TO PRODUCE ARESULTANT SIGNAL HAVING A FREQUENCY OF SAID COMPOSITE MINEDLY RELATED TOTHE FREQUENCY OF SAID COMPOSITE SIGNAL; MEANS COUPLED TO THE OUTPUT OFSAID RECEIVER AND THE OUTPUT OF SAID NETWORK TO PRODUCE A CONTREOLSIGNAL; MEANS TO COUPLE SAID CONTROL SIGNAL TO THE INPUT OF SAID NETWORKTO CONTROL THE FREQUENCY OF BOTH SAID SIGNAL SOURCES TO CAUSE THEFREQUENCY OF SAID RESULTANT SIGNAL TO FOLLOW THE FREQUENCY OF SAIDCOMPOSITE SIGNAL; AND MEANS TO COUPLE SAID TRANSMITTER TO SAID NETWORK.